1. Field of the Invention
This invention relates to the preparation and improvement of protein isolates. Specifically, this invention relates to the extraction of oilseed flour with solvent mixtures. Under some conditions certain azeotropic mixtures are preferred.
2. Description of the Prior Art
In order to better comprehend the state of the art it is necessary to understand certain facts. The currently abundant supply and relatively low per unit cost of oilseed proteins make them very attractive for improving the nutritional and functional performance of fabricated foods. Nevertheless, the vegetable proteins must meet certain unique color, taste, functional, and nutritional requirements indicated by the food industry. Currently the food industry produces three major classes of edible plant protein products to meet the specific food application need. These are flours, concentrates, and isolates. The Federal Register proposed certain standards of identify and definitions for these protein food products (see Vol. 39, No. 116, pp. 20891-20895, June 14, 1974). These pages indicate that a product having less than 65% protein (6.25 times the nitrogen content value) on moisture-free basis should be called "flour", a product having between 65% and 90% protein should be called "concentrate", and a product having above 90% protein should be called "isolate". The source of these protein products (cottonseed, soybean, etc.) must be identified because of the biochemical differences between the plant materials. Thus, according to the above definition the cottonseed protein isolate would be a protein product having at least 90% (6.25.times.N) and must be derived from cottonseed.
It is well known that "glanded cottonseed" contains about from 1% to 2% gossypol (lipid, moisture-free basis), and that it must be either physically removed or it must be bound to the protein by the application of heat and moisture to process it into meal (flour) for animal consumption. Hexane, the common solvent used for defatting oilseed, removes only trace amounts of gossypol during extraction. There are a number of patent and other disclosures in the literature which describe processes for removing free gossypol from the cottonseed to render it safe for animal consumption.
William H. King, et al. in their U.S. Pat. No. 2,950,198, disclose a process for removing unbound (free) gossypol and lipids without cooking the meats or the flakes of cottonseed by utilizing a mixture of acetone, hexane and water.
Walter A. Pons, et al. in their U.S. Pat. No. 3,557,168, disclose a process for extracting cottonseed with acetone or mixtures of acetone and hexane wherein most of the gossypol is removed.
Thomas P. Hensarling, et al. in their U.S. Pat. No. 3,941,764, disclose the use of a mixed solvent system that purportedly "extracts more lipid from cottonseed than hexane alone, without disrupting pigment glands or extracting amounts of pigment prohibitive to commercial applications". This mixed solvent system includes the use of acetic acid.
These processes of the prior art yield good results in the preparation of meal which is suitable for animal feed; however, it should be noted that the products are not acceptable for human consumption for one reason or another. The Pons method, for example, yields an inedible meal product because of its "catty odor", and when utilized to produce the isolate the method is unsatisfactory because of the additional disadvantage that the isolate produced from this meal is unacceptable dark colored. Theoretically, it is believed that when acetone is employed as the extracting solvent the acetone dimerizes the diacetone alcohol to yield mesityl oxide or its reaction with sulfhydryl moieties of the flour (proteins or polypeptides).
The King process, likewise, suffers from the same disadvantages. To produce an isolate that is untainted is impossible because, as indicated by those skilled in the art, acetone does not remove bound gossypol since it is not soluble therein. The Hensarling process denatures the protein in the meal so excessively that it cannot be processed into a protein isolate product. The particular application of acetic acid may well be the source of this problem.
It should be noted that efforts by processes of the prior art in producing light-color cottonseed protein isolate have not been fruitful. U.S. Pat. No. 3,814,748, discloses a process for improving the color of cottonseed protein isolate. The crux of said process lies in the performance and completion of the indicated steps within a prescribed period of less than about 30 minutes of time. This, of course, is obviously impractical, since in actua1 practice it has been difficult to complete the process steps within the recommended period of time in commercial production conditions.
In Food Technology 24:242 (1970) S. Gheyasuddin, et al describe a process of improving the color of sunflower protein, isolate by extracting the sunflower protein flour with 0.25% aqueous sodium sulfite at pH 10.5 and then extracting the protein isolate at its isoelectric point (pH 5) with about 50% aqueous isopropanol. However, M. K. Kim, et al (see Cereal Science Today 16:216 (1971) applied the Gheyasuddin process with glandless cottonseed flour but could not produce an improved color of the isolate.
The exact nature of the color-causing components has not been determined; however, preliminary investigations by the Southern Regional Research Center in New Orleans and Texas Agricultural and Mechanical University in College Station, Texas indicate that these compounds belong to the gossypol and flavonoid groups. More, it appears that these color components are intimately "tied" with the protein particles and therefore are not removed either during lipid extraction by hexane or subsequent alkaline protein extraction and water-washing of the protein isolate. Further research at the SRRC has indicated that gossypol is the major cause of color problem in cotton seed protein isolate preparation from cottonseed flour. Gossypol is also known to be extremely reactive and prone to oxidation during processing, especially in an alkaline medium (encountered during isolate preparation). Investigation has shown that nearly all the free and bound gossypol of the original starting cottonseed flour is transferred into cottonseed protein isolate as bound gossypol.
The addition of acetic acid in the extracting solvent mixture (instead of citric acid) was evaluated in this work, and it was determined that the use of acetic acid denatured the cottonseed protein excessively, to the point that the extracted flour was not suitable for isolate preparation. Also, the use of acetic acid did not result in significant improvement in the color of isolates.
The prior art teaches the improvement of flavor of oilseed protein products, specific concern for soybean oilseed being most evident. The presence of polar lipids (phospholipids) has been presumed to be the main cause of the flavor problems. Investigators of the flavor problems have dealt with the problems and with the reextraction of defatted oilseed flour by a second solvent mixture, as evidenced by U.S. Pat. No. 3,734,901, issued to L. P. Hayes, et al; A. C. Eldridge, et al in Cereal Chemistry 48:640, 1971; and D. H. Honig, et al in J. Food Science, Vol. 41, p. 642, 1976.